This invention relates to Global Positioning Systems (G.P.S.""s) and more particularly to a programmable waveform generator therefore.
As is well known in the art, the Global Positioning System (G.P.S.) includes 24 satellites orbiting the Earth in six orbit planes, each being inclined with respect to equatorial plane by 55xc2x0 and longitudinally offset from each other by 60xc2x0. It should be recognized that the present method is equally applicable for other positioning systems as well, such as GLONASS for example, wherein 24 satellites (21 operational and 3 spares) are inclined at 65xc2x0 angles and offset by 120xc2x0.
In G.P.S. each satellite broadcasts carrier signals on the same frequencies. Navigational data is spread by a clear acquisition (C/A) code and modulated on the L1 (1575.42 MHZ) in-phase channel. The navigational data is further spread by a P(Y) code and modulated on both the L1 quadrature and L2 (1227.60 MHZ) in-phase channels.
As is well known in the art, by measuring delay in the signals transmitted from several of these satellites, it is possible to triangulate ones position and further to correct for ionospheric induced, tropospheric induced and ephemeral/clock based errors, for example.
As both military and civilian requirements for G.P.S. rapidly change, a condition which will likely continue well into the 21st century, operational modifications will become increasingly necessary. In fact, requirements for the G.P.S. system have changed since the contract for the next generation satellites (Block IIF) was awarded. Many of these changing requirements revolve around the waveform structure and the desire to evolve it.
The lead time from award of contract to delivery (often 5-7 years) and from delivery to launch (often an additional 5-7 years) and then to eventual obsolescence (often greater than an additional 7.5 years) is such that once a signal structure is selected for a new block of satellites, to satisfy a current set of requirements, the navigation community is locked into that signal structure for several decades. Accordingly, there is a need for a flexible waveform generator aboard the G.P.S. satellites that can be reprogrammed in orbit, to facilitate continued evolution of the navigation system to meet changing operational requirements.
Conventional waveform generator architectures, wherein the entire composite signaling waveform is generated at baseband and then upconverted to the broadcast radio frequency (Rf), have been utilized in various types of communications systems (i.e. software radios) (See FIG. 1).
Although such an architecture can generally be used for communications systems, such an approach is not suitable for broadcast of the G.P.S. navigation signals from space. One limitation is the availability of space-qualified components that are fast enough to perform the required processing. This limitation results in an intermediate frequency that is too low to preclude harmonic interference in the resultant modulated signal. Another limitation is that any time jitter on the D/A converter results in additional phase noise on the composite output signal, which can violate the very stringent G.P.S. requirements. A third limitation is that bandpass filters are required for every mixer stage in the upconverter. When phase modulated signals are passed through these filters, they generate amplitude variations (xe2x80x9cringingxe2x80x9d) at every phase transition. These amplitude variations are problematic regarding satellite efficiency. Applying non-constant envelope signals to saturated amplifiers results in signal distortions that can impact navigation accuracy.
Of course, the use of saturated amplifiers on satellites is desirable due to the increased efficiency characteristics thereof.
Referring now to FIG. 1, therein is illustrated a conventional waveform generator 10 using the software radio method. Therein, a baseband composite signal is generated 12 and converted to an analog signal using digital to analog converters 14. The frequency of the analog signal is then up-converted to the desired L-band 18. As set forth though, due to the speed capabilities of current space qualified components (FPGA""s and D/A""s for example), interference can be caused by mixer harmonics when the first If frequency is too low in relation to signal bandwidth. Further, ringing of the bandpass filters in the mixer stages can also result in a non-constant envelope which results in further errors and reduced amplifier efficiency.
One of the requirements that has changed since the awarding of the G.P.S. Block IIF contract is the need for an additional military signal on the same L Band carrier as the C/A and P(Y) signals. Although several techniques are presently under consideration for combining the 3 signals, such as majority voting and hard limiting, the present invention utilizes the Interplex Modulation technique. Interplex Modulation is a very efficient technique for combining three or more signals to generate a constant envelope composite signal with minimal combining losses. FIG. 3 illustrates the conventional method for implementing Interplex Modulation and, as depicted therein, it requires the use of a linear modulator. A constant envelope composite signal is highly desirable so that a highly efficient saturated power amplifier can be utilized.
Referring now to FIG. 2, an alternative approach 20 would be to relocate the digital-to-analog interface 14 as illustrated therein. The first step would be to generate a baseband modulating signal 22 and convert that from digital to analog format 14. By converting at this step, a lower speed digital-to-analog converter 14 can be utilized because the modulating signal has less bandwidth than the composite signal. The analog signal would then be fed into a linear modulator 24 wherein it modulates the If frequency from a synthesizer 26 and up-converted 16 thereafter.
However, as will be evident to one skilled in the art, such an approach could still lead to ringing of the bandpass filters in the frequency up-converter 15 which in turn represents a significant risk in the development, certification and space use of such a linear modulator. Accordingly, not all of mentioned drawbacks would be overcome by such a system and method. Therefore, it is also desirable to avoid the inclusion of linear modulators.
FIG. 4 illustrates an interplex modulation technique 42 utilizing a combination of linear 44 and biphase 46 modulators. The output of this modulator 42 is able to produce a signal 48, however, no IM term is generated. Accordingly, the envelope of the output signal 48 is not constant and hence undesirable.
Referring now to FIG. 5, therein is illustrated a second interplex modulation technique 50 utilizing only biphase modulators 52. Again though, as illustrated for FIG. 4, the envelope of the resultant output signal 54 again is not sufficiently constant.
Referring now to FIG. 6, therein is illustrated a third interplex modulation technique 60 with a linear phase modulator 62 with the S3 channel in phase with S1 channel. However, as illustrated therein, a linear phase modulator 62 is required, therefore, such a configuration fails to remedy all of the aforementioned shortcomings.
Finally, referring to FIG. 7, therein is illustrated a fourth interplex modulation technique 70 utilizing a linear phase modulator 72 with the S3 channel in phase with the S2 channel. Again though, please note the inclusion of a linear phase modulator 72, as illustrated for FIG. 6.
Accordingly, each of these interplex modulation techniques (40, 42, 50, 60, 70) fails to remedy all of the aforementioned shortcomings of the prior art and hence satisfy each of the objects of the present invention.
A method for generating a global positioning signal from a space based craft including the steps of generating a plurality of binary modulating signals using a waveform generator; separately modulating an in-phase component of a desired carrier of the global positioning signal to be generated with at least a first binary, modulating signal selected from the plurality to generate at least one corresponding in-phase modulated signal component; separately modulating a quadrature component of the carrier with at least a second binary modulating signal selected from the plurality to generate at least one corresponding quadrature modulated signal component; and, combining the at least one in-phase modulated signal component and the at least one quadrature modulated signal component to generate the global positioning signal; wherein the global positioning signal has a constant envelope.